The invention relates to applying a toner print to a substrate, and measuring the amount of toner applied to the substrate by a printing machine.
In order to achieve a constant image quality in printing machines, it is necessary to control the printing process in such a way that the amount of toner in each case applied in the individual image areas results in the desired density and remains constant or substantially constant during continuous printing. In known printing machines, in the unfixed state of the toner, when the latter, for example, is still on an image carrier, such as an image cylinder, the toner density is measured and the printing process is controlled with the aid of this value. In the event of high densities of the toner, the relationship between the measured toner density and the amount of toner transferred to the substrate is no longer linear. As a result, in particular at high toner densities, which result from a high amount of toner applied to individual image areas, precise control of the printing process is not possible in every case.
In known printing machines, in the unfixed state of the toner, when the latter, for example, is still on an image carrier, such as an image cylinder, the toner density is measured and the printing process is controlled with the aid of this value. In the event of high densities of the toner, the relationship between the measured toner density and the amount of toner transferred to the printable surface is no longer linear. As a result, in particular at high toner densities which result from a high amount of toner applied to individual image areas, precise control of the printing process is not possible in every case.
It is therefore an object of the invention to provide that precise control or regulation of the amount of toner transferred to a substrate by a printing machine is ensured. In order to achieve the object, at least one print is applied to the substrate, for example paper, board, plastic or the like. Then, with the aid of a measuring device, the amount of toner transferred to the substrate is measured and the deviation between the actual amount of toner (ACT) applied in the area of the print and a desired amount of toner (DES) is determined. The difference between the actual amount of toner and the desired amount of toner is used to influence the printing process, the influence being exerted in such a way that the amount of toner transferred to the substrate to form the print is reduced or increased as a function of the respective deviation, so that the difference between the ACT amount of toner and the DES amount of toner becomes smaller. If the difference between the ACT amount of toner and the DES amount of toner is within a specific tolerance band, reducing or increasing the amount of toner applied to the substrate, and therefore exerting an influence on the control of the printing machine, is not required, so that the difference remains the same. The direct measurement of the amount of toner transferred to the substrate permits very precise control of the printing process, even if the print exhibits high toner densities. In order to optimize the printing process and to increase the measuring sensitivity further, provision is made to measure the toner coverage in advance; that is to say, the amount of toner transferred to the substrate at a specific setting of the printing process or of the printing machine. At least one test image is applied to a transport surface of a transport device for the substrate. Here, the term transport surface usually designates a part of a transport belt belonging to the transport device. As a result of applying the test image to the transport surface, the measurement step is independent of the condition of the substrate, for example moisture or thickness fluctuations. In addition, wastage of the substrate in the event of the wrong amounts of toner is avoided, since the printing of the substrate only begins when the difference between the ACT amount of toner and the DES amount of toner is within a specific tolerance band, that is to say has been controlled out to the desired extent.
In order to achieve the object, a printing machine is proposed, which includes a measuring device with the aid of which the amount of toner applied to the printable surface can be determined. By using the measuring device, exact determination of the amount of toner applied within an individual image area or within a print can be determined. This permits accurate control of the printing process, the measuring device being such that the respective amount of toner in a print can be determined accurately, both in the case of low and in the case of high toner densities.
The shape and/or size of at least one test image is advantageously matched to at least one electrode of a first capacitive sensor. Since the toners for different colors can have a different relative dielectric constant xcex5, at least one test image is preferably printed for each color and, for different amounts of toner, the characteristic curve xe2x80x9camount of toner per unit area in relation to the sensor output signalxe2x80x9d from the sensor is determined. For mixed colors, the characteristic curve of the sensor can likewise be determined. In a preferred embodiment, the measuring device has at least a first capacitive sensor which, for example, is designed as a proximity switch or distance measuring device. The construction and function of the capacitive sensor are known in the literature (for example from xe2x80x9cInduktive und kapazitive Sensorenxe2x80x9d (Inductive and capacitive sensors) by Andreas Schiff, Verlag Moderne Industrie (1989) and xe2x80x9cSensortechnikxe2x80x9d (Sensor engineering) by Harry Herold, Huthig Verlag Heidelberg (1993)), of which the content relating to the construction and the function of the capacitive sensor is made the subject of this application. In the case of these devices, which include a proximity switch or a distance measuring device, for example, the change in capacitance of a capacitor is measured. Given a constant electrode area, the capacitance may be changed by enlarging the distance between the two electrodes of the capacitor or by changing the effective relative dielectric constant e of the medium between the electrodes. In a particularly preferred embodiment, the two capacitor plates are formed by the first capacitive sensor and the measured object, in particular the substrate with toner applied.
In an advantageous exemplary embodiment, provision is made for the first capacitive sensor to be arranged in a fixed position at a distance from and opposite a transport surface of a transport device for the substrate. On the basis of this arrangement, the substrate and the toner applied thereto, as they move through the interspace between the sensor and the transport surface on which the substrate rests, in each case effect a change in the relative dielectric constant xcex5 of the medium between the capacitor plates (sensor and substrate). This change effects a corresponding change in the sensor output signal and can be used to control the printing process. With the aid of the control system, the amount of toner in each case applied to the substrate is increased or, if appropriate, reduced.
In order to carry out the measurement of the amount of toner on the substrate, provision is made in a first exemplary embodiment for the toner to be fixed on the substrate previously by, for example, being melted on in a known way. In another embodiment, the measurement of the amount of toner is carried out with the toner transferred to the substrate but in the unfixed state.
The aforementioned xe2x80x9cmeasured objectxe2x80x9d is initially the transport surface, which for example, is formed by a transport belt. As soon as the print with a substrate having a specific amount of toner passes into the interspace between the sensor and the transport surface, the xe2x80x9cmeasured objectxe2x80x9d is formed by the substrate. It remains to be recorded that, with the aid of the first capacitive sensor, the distance between the sensor arranged in a fixed position and the substrate is measured. The amount of toner applied to the substrate results in a specific toner density for the print having a constant area, the distance between the sensor and the substrate becoming smaller as the toner density increases. In other words, the greater the amount of toner applied to an individual printing area, the higher the amount of toner projects beyond the surface of the substrate, and the lower is the distance, measured with the aid of the capacitive sensor, between the sensor and the substrate or the print.
In a preferred embodiment, the first capacitive sensor is triggered in such a way that it measures the distance between itself and the measured object (transport surface or substrate) at the time at which the test image is located precisely under it or in the interspace bounded by the sensor and the transport surface. In order to control the first capacitive sensor, signals from the existing control devices belonging to the printing machine, for example, can be used, which simplifies their construction.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.